Since from the past, an insulated wire in which a conductor is covered with an insulating film has been used in an electrical coil for various kinds of electrical equipment, such as motors and transformers. The insulated wire for forming the electrical coil is required to have adhesion to the conductor, electrical insulating property, and heat resistance. Especially in recent years, electrical equipments for aerospace use, electrical equipments for aircrafts, electrical equipments for nuclear power, electrical equipments for energy, and electrical equipments for automobiles, are required not only to be reduced in size and weight thereof but also to have higher performance. For example, rotating electrical machines such as a motor and transformers, are required to have a higher output power ever than before.
The rotating electrical machines are produced by pushing an insulated wire that is wound around a core into a slot. Since the insulated wire is pushed into this slot as many as possible, a high pressure is applied to an enameled wire. Therefore, the coating film of the insulating film should be made strong. However, since there are limitations on the strength of resins, it is necessary to make a structure which absorbs the impact.
In addition, requirement is increasing for a further thinner insulating film of the insulated wire. Thus, an improvement in dielectric breakdown voltage of the insulated wire is considered to be indispensable. Further, an insulated wire having a thin insulating film is required for being capable of reducing damage of the insulating film, which damage is caused upon pushing the insulated wire into the slot.
Further, a corona discharge may occur between the insulated wire and the slot and between insulated wires themselves, when a high voltage is applied at the time of operation of the rotating electrical machine. In a case where the applied voltage is not so high, requirement of corona-discharge resistance in an insulated wire was not so high. However, because a high voltage is applied in a rotating electrical machine with a high output power, an insulated wire is required, which is excellent in the corona-discharge resistance and high in the partial discharge inception voltage.
To improve the partial discharge inception voltage of an insulated wire, thickening of the insulating film is assumingly effective. However, in view of requirement of the film being thinner in the insulated wire, thickening of the insulating film is difficult. Usually, an insulated wire is produced by applying and baking a varnish of resin onto a conductor repeatedly. Since for thickening the insulating film, the number of times for passing through a baking furnace increases in a production process thereof, a coating film composed of copper oxide on a copper conductor surface is being thicker, due to this, adhesion between the conductor and the insulating film is lowered.
As another method to improve the partial discharge inception voltage of the insulated wire, a resin low in dielectric constant is used in the insulating film. However, such a use of the resin low in the dielectric constant is difficult, because the resin is usually low in a surface free energy and poor in the adhesion with the conductor.
Further, an insulated wire is proposed, which has an improved corona-discharge resistance, by blending particles into the insulating film. For example, there are proposals of an insulating film containing particles composed, for example, of alumina, silica, and chromium oxide (see, Patent Literatures 1 and 2) and an insulating film containing particles composed, for example, of nitrogen carbide and silicon nitride (see, Patent Literature 3). In these insulated wires, an erosion deterioration due to corona discharge is tried to be reduced, owing to the insulating film containing the particle. However, in the insulated wire having the insulating film containing these particles, flexibility of the film lowers to result in that a film surface may become rough in many cases. Due to this rough film surface, it is difficult to push the insulated wire into a slot. As a result, depending on the case, the insulated wire is poor in the abrasion resistance and the insulating film is apt to be damaged.
Furthermore, it has been suggested to use batch type foaming of a thermoplastic resin in order to decrease the dielectric constant of an insulated wire (see Patent Literature 4). For example, carbon dioxide is incorporated into a polyphenylene sulfide resin under high pressure conditions to cause the resin to foam. However, this method has a problem that bubbles within the thermoplastic resin collapse as a result of pressure molding, and the heatproof temperature is low.
Furthermore, it has been suggested to add a water-soluble alcohol or ether, or water to a varnish that foams a polyimide precursor; however, in this technique, it is known that since a low boiling alcohol or ether is used, an additive preferentially reacts with the polyimide precursor, a molecular weight is reduced, and sufficient heat resistance cannot be obtained (see Patent Literature 5).
Furthermore, Patent Literature 6 describes a technology for an insulated wire in which an insulating coating film is formed from a cured product of a thermosetting resin composition containing a thermoplastic resin and the insulating coating film has fine pores. However, in this technology, the thermosetting resin contains no bubbles. Bubbles are formed inside the thermoplastic resin particles in a network structure formed by applying a varnish of thermosetting resin containing a thermoplastic resin and baking the resin varnish.